U.S. Pat. No. 3,277,138 discloses a two-step process for chlorinating aromatic carbocyclic isocyanates by first chlorinating the isocyanate at a temperature of from -20.degree. to 150.degree.C. in the absence of the catalyst, and then further chlorinating the resulting product at from 210.degree.-250.degree.C. in the presence of the catalyst. This process, however, has the disadvantage of being long and time-consuming.